Since styrene resins represented by ABS resins have excellent mechanical properties, moldability and electric insulation properties, they are used in a wide range of fields such as various parts for home electric appliances, office automation equipment and automobiles. However, since most of the plastics comprising styrene resins are flammable, various techniques have been proposed for achievement of flame retardancy in view of safety.
In general, a method wherein a chlorine flame retardant and a bromine flame retardant having high flame retardancy efficiencies and antimony oxide are blended in a resin is employed for achieving flame retardancy. However, since this method uses a large amount of flame retardants for prevention of dropping of flame sources (dripping) upon combustion, the method has a drawback in that the mechanical properties and the heat resistance of the resin composition tend to be low, and the method also has a problem in that toxic gases are produced due to decomposition of halogen compounds upon molding and upon combustion. From the viewpoint of recent environmental problems, non-halogen resins containing neither chlorine flame retardant nor bromine flame retardant are demanded.
Examples of non-halogen flame retardants include phosphorus flame retardants, and representative examples of phosphorus flame retardants which have been conventionally commonly employed include phosphoric acid esters. Examples of the methods disclosed so far include a method wherein polyphosphate is added to a styrene resin (Patent Document 1), method wherein a phosphoric acid ester having a specific structure is added to a rubber reinforced styrene resin (Patent Document 2), and a method wherein a liquid phosphoric acid ester is added to a styrene resin (Patent Document 3). However, since styrene resins are extremely flammable resins, the flame retardancy effects of phosphoric acid esters are low, and, in compositions obtained by the methods described in Patent Documents 1 to 3, blending of a large amount of a phosphoric acid ester is necessary to give flame retardancy to a styrene resin. This leads not only to decreased mechanical properties but also to bleeding out of the phosphoric ester, occurrence of mold fouling upon molding, and generation of gas upon molding, which are problematic.
On the other hand, as a method for solving the above problems, a method using a hydroxyl-containing phosphoric acid ester is disclosed in Patent Document 4. However, since hydroxyl-containing phosphoric acid esters also have low flame retardancy effects, the above problems could be hardly solved. Patent Document 5 discloses a method wherein a novolac phenol resin as a carbonized layer-forming polymer and a compound having a triazine skeleton are added to a hydroxyl-containing phosphoric acid ester for further improving the flame retardancy. This technique also failed to solve the problem of deteriorating the original mechanical properties, impact resistance and moldability of the styrene resin. Further, since phenol resins are materials having extremely low light resistance, the light resistance of the obtained resin composition decreases, which is also problematic.
Further, Patent Document 6 discloses a method wherein red phosphorus having a high flame retardancy effect is used as a non-halogen flame retardant, and a phenol resin having carbonized layer-forming capacity is added as a flame retardant aid. Although this technique can give flame retardancy, the original mechanical properties, impact resistance and moldability of the styrene resin are deteriorated. The method also has a problem in that red phosphorus makes the molded product colored with the color of red phosphorus.
Further, in Patent Document 7, it was discovered that, by blending a specific phosphoric acid ester compound and a specific phosphorous acid ester compound as flame retardants at specific ratios in a styrene resin, a flame-retardant resin composition having high flame retardancy as well as excellent mechanical strength, impact resistance and moldability can be obtained. However, the flame retardancy was not sufficient in some cases.
On the other hand, although there have been cases where an aromatic carbonate oligomer was added for improvement of the external appearance of a polycarbonate resin composition filled with a filler such as a glass fiber (Patent Document 8) or for improvement of the fluidity of a polycarbonate resin composition (Patent Document 9), the fact that addition of an aromatic carbonate oligomer to a styrene thermoplastic resin composition contributes to improvement of the flame retardancy of the resin composition has not been discovered.